postgres: fix the port override properly — a twin variable, not a raw substitution

The first commit on this branch embedded ${seat:mesh-store:5432} directly
inside MESH_PROVISION_POSTGRES's URL. That placeholder resolves to an empty
string whenever mesh-store is on its own default port (novox/hq
internal/catalogue/seat_into.go: 'the mesh raised on the catalogue's own
ports never gives them a setting at all') -- which produces a malformed
connection string (host:/postgres) on exactly the common case, a fresh,
non-adopted mesh. It only worked here because novox's mesh-store happens to
be adopted at a non-default port.

mesh-controller's own manifest already has the right shape for this --
internal/envfile/port.go's NAME / NAME_PORT twin, composed by
mesh-controller's Placed(): the base value keeps its own port; a separate
_PORT variable carries the override, spliced in only when it says
something, and left alone -- not a fault -- when it's an unfilled
placeholder (a manifest ahead of the running controller).

Reverted the manifest to its original base value, added
MESH_PROVISION_POSTGRES_PORT as the twin, and taught client.ts (the only
consumer -- both tools/ and provisioner/ import it) the same precedence
envfile.Placed uses. Checked: no other file in the module reads
MESH_PROVISION_POSTGRES directly.

modules/postgres/module.json

@@ -97,7 +97,8 @@
         "/var/lib/postgres/superuser.secret:/run/secrets/superuser:ro"
       ],
       "env": {
-        "MESH_PROVISION_POSTGRES": "postgres://postgres@127.0.0.1:${seat:mesh-store:5432}/postgres?sslmode=disable",
+        "MESH_PROVISION_POSTGRES": "postgres://postgres@127.0.0.1:5432/postgres?sslmode=disable",
+        "MESH_PROVISION_POSTGRES_PORT": "${seat:mesh-store:5432}",
         "MESH_PROVISION_PASSWORD_FILE": "/run/secrets/superuser",
         "MESH_BROKER_FILE": "/run/secrets/broker",
         "MESH_RECEIVES": "/var/lib/postgres/grants/mesh.json"